INNER CRUISE/EARTH FLYBY

After traveling in space for two years, Juno swings by Earth once more, in October 2013. It will use Earth’s gravity to help propel
itself toward Jupiter– a maneuver sometimes called a gravitational slingshot.

WHY FLY BY?

When a spacecraft flies near a planet, both objects feel each other’s gravitational pull. But because a spacecraft is so much smaller, it feels a big tug while the massive planet hardly feels a thing. During the Earth flyby, called a gravity assist, Juno steals a tiny bit of our planet’s enormous momentum in its orbit around the sun, giving the spacecraft enough energy to reach beyond the asteroid belt and travel all the way to Jupiter.

When Juno launched, there was not an available rocket powerful enough to send such a heavy spacecraft directly Jupiter, so the flyby maneuver is an essential part of the mission. The Atlas V rocket provided half of the boost Juno needs to reach Jupiter, and the Earth flyby provides the rest. It’s the energy of an extra rocket launch, without the extra rocket.

The flyby is also important because it acts as a rehearsal for the Juno team to test the instruments and practice certain operations at Earth before the real thing at Jupiter. After its brief visit, Juno will cruise for three more years, receiving routine checkup calls from engineers. The spacecraft will report its health status at least once a week and engineers will turn on and test its science instruments at least once a year.

During its flyby rendezvous, Juno will spend about 20 minutes within Earth’s shadow – the only eclipse Juno ever experiences after launch. At closest approach, the spacecraft will come within 350 miles (560 kilometers) of Earth’s surface.

Interact with the hotspots below to learn more.

HOW DO YOU POWER A SIX-YEAR JOURNEY?

Juno is powered by three huge solar arrays that are arranged on the spacecraft like a windmill. The arrays have to be big because Jupiter is five times farther from the sun than Earth, which means the sunlight is 25 times weaker. Despite their size, the arrays will produce only about 450 watts of power at Jupiter – only enough to run four standard light bulbs.

How fast can Juno go?

When arriving at Jupiter, the planet's gravity pulls in Juno faster and faster until the spacecraft reaches a speed over 250,000 kilometers per hour (150,000 miles per hour) with respect to Earth – making it one of the fastest human-made objects ever. When it arrives at Jupiter, it slams on the brakes, firing its main engine in reverse. After slowing down, Juno can then enter Jupiter orbit.

Juno will cover 2.8 billion kilometers (nearly 1.8 billion miles) during its long, looping voyage. That’s 19 times farther than the distance between Earth and the Sun, and 15 times farther than the closest distance between Earth and Jupiter. If Juno were to fly at the speed of a commercial jet, it would take 342 years to complete its journey!

Long communication delays makes steering Juno with a joystick impossible.

Driving the spacescraft

After launch, Juno is on its own. Because the spacecraft will be so far away, instant communication is impossible. Mission controllers can steer Juno by firing its main engine or thrusters, but they have to send the commands ahead of time. Jupiter is five times farther from the Sun than Earth, so even though radio signals travel at the speed of light – nothing in the universe can go faster – it still takes 45 minutes to send a message. It’s another 45 minutes to wait for a reply.

Engineers make long-distance calls to make sure the spacecraft is working.

How do we keep Juno healthy?

Juno received frequent checkups during the first few months after launch. After that, engineers will check on Juno at least once a week. They’ll also perform routine maintenance on the science instruments four times a year. The Earth flyby in 2013 will give the team a chance to test the instruments and practice taking data.

Engineers will also perform several tests on Juno’s propulsion, navigation, and pointing control systems en route to Jupiter. In particular, the deep-space maneuver that sets up Juno’s Earth flyby will serve as a test of the main engine.

Protecting Juno

Space is a harsh and unforgiving environment. Tiny meteors can strike an unprotected spacecraft like bullets. Extreme temperatures mean it’s either scorching hot or bitterly cold, and without air to circulate, there’s nothing to prevent Juno from overheating or freezing. Another danger is Jupiter’s belt of speeding charged particles – a potent source of radiation.

To keep Juno safe, engineers cover the spacecraft with a shiny, protective layer that shields it against tiny meteors and prevents Juno from getting too hot or cold. This thin skin is made out of material that allows electricity to flow around Juno. The environment around Jupiter is littered with charged particles, and if electricity isn’t allowed to flow, charge can build up on the spacecraft and unleash a dangerous spark.